11,489 research outputs found
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
The frequencies of transverse-optical phonons in KNbO and
KTaO are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the symmetry. The TO frequencies in KTaO are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO.Comment: 8 pages + 1 LaTeX figure, Revtex 3.0, SISSA-CM-94-00
Stress corrosion cracking of titanium alloys
The effect of hydrogen on the properties of metals, including titanium and its alloys, was investigated. The basic theories of stress corrosion of titanium alloys are reviewed along with the literature concerned with the effect of absorbed hydrogen on the mechanical properties of metals. Finally, the basic modes of metal fracture and their importance to this study is considered. The experimental work was designed to determine the effects of hydrogen concentration on the critical strain at which plastic instability along pure shear directions occurs. The materials used were titanium alloys Ti-8Al-lMo-lV and Ti-5Al-2.5Sn
The Density of Lyman-alpha Emitters at Very High Redshift
We describe narrowband and spectroscopic searches for emission-line star
forming galaxies in the redshift range 3 to 6 with the 10 m Keck II Telescope.
These searches yield a substantial population of objects with only a single
strong (equivalent width >> 100 Angstrom) emission line, lying in the 4000 -
10,000 Angstrom range. Spectra of the objects found in narrowband-selected
samples at lambda ~5390 Angstroms and ~6741 Angstroms show that these very high
equivalent width emission lines are generally redshifted Lyman alpha 1216
Angstrom at z~3.4 and 4.5. The density of these emitters above the 5 sigma
detection limit of 1.5 e-17 ergs/cm^2/s is roughly 15,000 per square degree per
unit redshift interval at both z~3.4 and 4.5. A complementary deeper (1 sigma
\~1.0 e-18 ergs/cm^2/s) slit spectroscopic search covering a wide redshift
range but a more limited spatial area (200 square arcminutes) shows such
objects can be found over the redshift range 3 to 6, with the currently highest
redshift detected being at z=5.64. The Lyman alpha flux distribution can be
used to estimate a minimum star formation rate in the absence of reddening of
roughly 0.01 solar masses/Mpc^3/year (H_0 = 65 km/s/Mpc and q_0 = 0.5).
Corrections for reddening are likely to be no larger than a factor of two,
since observed equivalent widths are close to the maximum values obtainable
from ionization by a massive star population. Within the still significant
uncertainties, the star formation rate from the Lyman alpha-selected sample is
comparable to that of the color-break-selected samples at z~3, but may
represent an increasing fraction of the total rates at higher redshifts. This
higher-z population can be readily studied with large ground-based telescopes.Comment: 7 pages, 5 encapsulated figures; aastex, emulateapj, psfig and lscape
style files. Separate gif files for 2 gray-scale images also available at
http://www.ifa.hawaii.edu/faculty/hu/emitters.html . Added discussion of
foreground contaminants. Updated discussion of comparison with external
surveys (Sec. 5 and Fig. 5). Note: continuum break strength limits (Fig. 3
caption) are correct here -- published ApJL text has a sign erro
The Eight Forms of Corrosion & the Corrective Measures
Corrosion has been justifiably attributed to cause more loss in money than any other single factor. Corrosion has also been an important factor in causing some outstanding cases of failure.
Valuable information for the solution of a corrosion failure can often be obtained through careful observation of the corroded test specimens or, failed equipment. Corr-osion can be classified by the eight forms in which it manifests itself, the basis for this classification being the appearance of the corroded metal. Some of these eight forms are unique and distinct but all of them are to varying degrees interrelate
Probing the evolution of the near-IR luminosity function of galaxies to z ~ 3 in the Hubble Deep Field South
[Abridged] We present the rest-frame Js-band and Ks-band luminosity function
of a sample of about 300 galaxies selected in the HDF-S at Ks<23 (Vega). We use
calibrated photometric redshift together with spectroscopic redshift for 25% of
the sample. The sample has allowed to probe the evolution of the LF in the
three redshift bins [0;0.8), [0.8;1.9) and [1.9;4) centered at the median
redshift z_m ~ [0.6,1.2,3]. The values of alpha we estimate are consistent with
the local value and do not show any trend with redshift. We do not see evidence
of evolution from z=0 to z_m ~ 0.6 suggesting that the population of local
bright galaxies was already formed at z<0.8. On the contrary, we clearly detect
an evolution of the LF to z_m ~ 1.2 characterized by a brightening of M* and by
a decline of phi*. To z_m ~ 1.2 M* brightens by about 0.4-0.6 mag and phi*
decreases by a factor 2-3. This trend persists, even if at a less extent, down
to z_m ~ 3 both in the Js-band and in the Ks-band LF. The decline of the number
density of bright galaxies seen at z>0.8 suggests that a significant fraction
of them increases their stellar mass at 1<z<2-3 and that they underwent a
strong evolution in this redshift range. On the other hand, this implies also
that a significant fraction of local bright/massive galaxies was already in
place at z>3. Thus, our results suggest that the assembly of high-mass galaxies
is spread over a large redshift range and that the increase of their stellar
mass has been very efficient also at very high redshift at least for a fraction
of them.Comment: 18 pages, 21 figures, Accepted for publication in MNRA
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